One of the principal problems encountered in the drilling of, and production from, deep wells, is the high pressure found in these formations. Modern deep gas wells are known to operate at bottom hole pressures of up to 30,000 psi. Pressures of magnitudes in excess of 20,000 psi present a set of qualitatively different engineering problems for wellhead seal design. In addition to the inherent problem of sealing with minimal leakage, high bottom hole temperatures and corrosive gases compound the high pressure sealing problem. Because of the depths involved and the complexity of production equipment, equipment for these ultrahigh pressures must be designed such that it can be tested after being set in place prior to actual production. Improper seating must be detected and corrected at a stage when dismantling can still be done relatively quickly and inexpensively.
In the prior art, Belleville seals are commonly known to operate satisfactorily at moderate pressures. However, even at moderate pressures, Belleville seals are generally not elastic enough to seal against substantial pressure fluctuation or pressure reversal without loss of the low pressure seal. They will therefore not permit the application of test pressure from a direction opposite the operating pressure.
When installing a casing hanger, once the casing hanger is landed in the hole, the seals are generally set by the weight of the casing string suspended from the casing hanger. When the next head above the hanger is to be installed, the integrity of the intermediate seals between the hanger and head must be tested. Most prior art designs do not possess the capability of pressure testing from a direction other than the working pressure side. In applying test pressure on unidirectional metal seals, mandrel and bore are frequently damaged since Belleville seals will "coin" or dig into the surface thus rendering the seal inoperative under pressure fluctuations. To achieve both a low pressure (setting) seal and high pressure (working pressure) seal, many prior art designs employ a composite elastomeric/metal seal or a set of Chevron elastomeric packings in addition to the main metal seal between which the test pressure for the metal seal is applied. Sour gas environments, however, high pressures and operating temperatures of up to 350.degree. F. severely restrict the applicability of such elastomeric seals or render them one-time test seals only.
In related prior art, softer metals have often been used for frustoconically tapered seals. For example, U.S. Pat. No. 1,323,660 to H. C. Thrift discloses a well capping device consisting of a sleeve adapted to fit over the casing. The sleeve is pressed against the casing by means of wedge-shaped slips whose inner faces are serrated, forming arcuate teeth for engaging the casing wall. Between the wedge-shaped slips and a flared collar at the bottom of the sleeve are positioned several frustoconically shaped rings made of very soft metal, such as lead. In case of an impending blowout, the casing pressure will cause the conical ring to be compressed between the slips and the collar. Being of soft metal, the rings will be flattened, thereby forming a close fitting joint between the casing and the sleeve. Lead, however, is a metal practically devoid of tenacity, ductility and elasticity and would therefore not be capable of sustaining much circumferential stress and elongation and would not return toward its original shape upon release of the load. Unless the initial clearance between such a lead seal ring and the inner and outer surface with which it is to seal closely approaches zero, the lead ring will fracture in hoop tension upon imposition of load. In order for sealing pressure enhancement along the radial surfaces to come into play, the hoop stresses must be able to elastically expand the rings in a radial direction.
U.S. Pat. No. 2,090,956 to Wheeler teaches the use of a series of frustoconically shaped packing rings, made not of metal, but of some porous material. These rings may be compressed longitudinally between similarly shaped adapter rings. Between the bevelled faces of the adapter rings, the soft packing rings become flattened and radially pressed against the sealing surfaces.
The downhole packer disclosed in U.S. Pat. No. 2,120,982 to Layne uses a lead sleeve to contact the inside diameter of a concentric casing string. An alternative embodiment teaches the use of interlocking frustoconical wedge rings made of lead which are compressed and enhance the sealing contact between the inner and outer diameter surfaces when loaded by the make-up pressure supplied by screwing together the liner and casing. There is a substantial gap, however, between liner and casing such that the lead wedge must become outwardly flared for sealing to take place. These wedges then act more in the fashion of a lip seal and are therefore strictly undirectional seals.
U.S. Pat. No. 2,135,583 to Layne describes a combination packer which uses a soft lead seal to back up a fabric or a second soft metal packing for increased reliability. The packing is set by the weight of the string of pipe and compressed to a generally frustoconical shape.
U.S. Pat. No. 3,347,319 to Littlejohn pertains to methods and apparatus for hanging large diameter casing. An interior hanging ring of generally triangular cross section is welded to the interior of the larger string of casing. A matingly tapered exterior hanging ring of generally frustoconical shape is affixed to the exterior of the smaller length of casing. The two main purposes mentioned in the patent for this metal-to-metal seal are to afford a projection on which the succeeding length of casing may be hung and to reinforce the casing to prevent its failure by outward pressure. No sealing function seems to be intended or achieved.
U.S. Pat. No. 3,436,084 to Courter discloses a packer with an elastomeric packer element which is contained to substantially eliminate the flow of the elastomeric material under pressure. A series of arcuate segments along one circumferential edge of the deformable packing element are made by spaced vertical cuts. The segments have metal or hard tough resin faced plates molded onto their mating vertical end faces and connecting reinforcing slideable pins through the face plates prevent the packing element from flowing longitudinally under pressure when the packing element itself is forced outwardly.
U.S. Pat. No 3,797,864 to Hynes et al. describes a well casing hanger with a seal deformable into sealing engagement with opposed cylindrical walls of the casing hanger body and another body upon actual compression of the seal. The seal is a compound of a cylindrical elastomeric body and metallic skirt or end rings on the corners of the elastomeric element. The end rings are provided with marginal lips which are deformed oppositely into metal-to-metal sealing engagement with the cylindrical walls. The metal rings thus appear to be acting primarily as an anti-extrusion device for the elastomeric element.
U.S. Pat. No. 3,902,743 to Martin pertains to a retractable support shoulder arrangement providing a split ring seat that facilitates running maximum size downhole tools through the upper access opening of the casinghead during drilling operations. In its extended position the split ring seat element provides an essentially full circle seating surface for firmly and properly supporting a casing hanger or other device in the head. In that position it presents generally frustoconically tapered seating surfaces which do not appear to have any sealing function.
The present invention overcomes the problems and deficiencies of the prior art and specifically permits a bidirectional application of pressure whereby the seal may be tested from a direction opposite the operating pressure. Other advantages of the present invention will be apparent from the following description.